An electric power steering apparatus that energizes a steering apparatus of a vehicle by using a rotational torque of a motor as an assist torque, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. And then, in order to supply a current to the motor so that the motor generates a desired torque, an inverter is used in a motor drive circuit.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on a steering torque T detected by the torque sensor 10 and a velocity Vs detected by a velocity sensor 12, and controls a current supplied to the motor 20 based on a voltage command value E obtained by performing compensation and so on with respect to the current command value in a current control section. Moreover, it is also possible to receive the velocity Vs from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque T detected by the torque sensor 10 and the velocity Vs detected by the velocity sensor 12 are inputted into a current command value calculating section 101 that calculates a current command value Iref1. The current command value calculating section 101 decides the current command value Iref1 that is a desired value of the current supplied to the motor 20 based on the inputted steering torque T and the velocity Vs and by means of an assist map or the like. The current command value Iref1 is added in an addition section 102A and then is inputted into a current limiting section 103 as a current command value Iref2. A current command value Iref3 limited the maximum current is inputted into a subtraction section 102B, and a deviation Iref4 (=Iref3−Im) between the current command value Iref3 and a motor current value Im that is fed back, is calculated. The deviation Iref4 is inputted into a current control section 104 that performs PI control and so on. The voltage command value E that a characteristic improvement is performed in the current control section 104, is inputted into a PWM control section 105. Furthermore, the motor 20 is PWM-driven through an inverter 106 serving as a drive section. The current value Im of the motor 20 is detected by a current detector 106A within the inverter 106 and fed back to the subtraction section 102B. In general, the inverter 106 uses EFTs as switching elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensation section 110 is added in the addition section 102A, and compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation section 110 adds a self-aligning torque (SAT) 113 and an inertia 112 in an addition section 114, further adds the result of addition performed in the addition section 114 and a convergence 111 in an addition section 115, and then outputs the result of addition performed in the addition section 115 as the compensation signal CM.
In the case that the motor 20 is a 3-phase (A-phase, B-phase and C-phase) brushless motor, details of the PWM control section 105 and the inverter 106 become a configuration such as shown in FIG. 3. The PWM control section 105 comprises a duty calculating section 105A that calculates PWM duty command values D1˜D6 of three phases according to a given expression based on the voltage command value E, and a gate driving section 105B that drives each gate of FET1˜FET6 by the PWM duty command values D1˜D6 to turn ON/OFF. The inverter 106 comprises a three-phase bridge having top and bottom arms comprised of A-phase high-side FET1 and A-phase low-side FET4, top and bottom arms comprised of B-phase high-side FET2 and B-phase low-side FET5, and top and bottom arms comprised of C-phase high-side FET3 and C-phase low-side FET6, and drives the motor 20 by being switched ON/OFF based on the PWM duty command values D1˜D6.
Moreover, a PWM duty command value for the A-phase is set as Da, a PWM duty command value for the B-phase is set as Db, and a PWM duty command value for the C-phase is set as Dc.
In such a configuration, although it is necessary to measure a drive current of the inverter 106 or a motor current of the motor 20, as one of request items of downsizing, weight saving and cost-cutting of the control unit 100, it is singulation of the current detector 106A (one-shunt type current detector). A one-shunt type current detector is known as the singulation of a current detector, and for example, the configuration of the one-shunt type current detector 106A is shown in FIG. 4. That is to say, a one-shunt resistor R1 is connected between the bottom arm of the FET bridge and ground, a fall voltage that is caused by the shunt resistor R1 when a current flowed in the FET bridge, is converted into a current value Ima by an operational amplifier 106A-1 and resistors R2˜R4, and an A/D converting section 106A-2 A/D-converts the current value Ima at a given timing and then outputs a current value Im that is a digital value.
FIG. 5 shows a wiring diagram of a power supply (a battery), the inverter 106, the current detector 106A and the motor 20. As one example, FIG. 6 shows a current pathway (indicated by a dashed line) during a state that the A-phase high-side FET1 is turned ON (the A-phase low-side FET4 is turned OFF), the B-phase high-side FET2 is turned OFF (the B-phase low-side FET5 is turned ON), and the C-phase high-side FET3 is turned OFF (the C-phase low-side FET6 is turned ON). Further, as another example, FIG. 7 shows a current pathway (indicated by a dashed line) during a state that the A-phase high-side FET1 is turned ON (the A-phase low-side FET4 is turned OFF), the B-phase high-side FET2 is turned ON (the B-phase low-side FET5 is turned OFF), and the C-phase high-side FET3 is turned OFF (the C-phase low-side FET6 is turned ON). It is clear from these current pathways of FIG. 6 and FIG. 7 that the total value of phases that high-side FET is turned ON, appears in the current detector 106A as a detected current. That is, it is possible to detect an A-phase current in FIG. 6, and it is possible to detect the A-phase current and a B-phase current in FIG. 7. This is the same as in the case that the current detector 106A is connected between the top arm of the inverter 106 and the power supply.
In this way, the current detector 106A detects the motor current when any one of phases is turned ON and two phases are turned ON, and by utilizing a characteristic that the sum of currents of three phases is equal to zero, it is possible to detect each-phase currents of the A-phase, the B-phase and the C-phase. By utilizing the above-described characteristic, the current detection based on a one-shunt type current detector can detect each-phase currents. In this case, in order to perform the current detection while removing noise components such as rigging noises flowing in the current detector soon after the FET is turned ON, a certain period of time becomes necessary. That is, in order to detect each-phase currents by the one-shunt type current detector, by making a status that keeps an ON-state of PWM of intended phases for a given period of time and performing the current detection, each-phase motor currents are detected. Therefore, although it is necessary to maintain a one-phase ON-state and a two-phase ON-state for a necessary time for the current detection, in the case that each-phase duty command values become equal to each other, a problem that it is impossible to secure that a duration time, arises.
As background arts for solving such a problem, there are an apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-118621 (Patent Document 1) and an apparatus disclosed in Japanese Patent Application Laid-Open No. 2007-112416 (Patent Document 2).
The apparatus disclosed in Patent Document 1, when determining that the current detection is impossible, shifts the PWM phase by moving the phase of the PWM signal just a given amount, and secures a time becoming “PWM-ON” for the current detection necessary time to perform the current detection. That is, the apparatus disclosed in Patent Document 1, comprises a number of switching determining means that determines the number of switching elements of top arms that are turned on is an odd number or an even number in the case that a current detection advisability determining means determines that the current detection is impossible, and a phase moving means that moves the PWM signal of a given phase just a given amount to a given moving direction in the case that the number of switching determining means determines that it is an even number and moves the PWM signal of the given phase just the given amount to the opposite direction in the case that the number of switching determining means determines that it is an odd number, or moves the phase of a PWM signal having a maximum value of duty among each-phase PWM signals just the given amount to the given moving direction and moves the phase of a PWM signal having a minimum value of duty among each-phase PWM signals just the given amount to the opposite direction.
Further, the apparatus disclosed in Patent Document 2, shifts the PWM phase by that each-phase PWM signals have carriers with different phases respectively and perform the PWM output, and secures a time becoming “PWM-ON” for the current detection necessary time to perform the current detection. That is, in the apparatus disclosed in Patent Document 2, on a current pathway between a motor drive circuit and ground, a single current sensor for detecting a current value flowing the current pathway is provided, by shifting the phase of a saw-tooth wave for generating each-phase PWM signals and shifting timings of falling to a low level of each-phase PWM signals, based on the output signal of the single current sensor in a period until the elapse of a certain time starting from the timing of falling to the low level of V-phase PWM signal, the value of U-phase current flowing the motor can be obtained.
Both of the apparatus disclosed in Patent Document 1 and the apparatus disclosed in Patent Document 2, detect each-phase currents of the motor by a one-shunt type current detector by shifting the PWM phase so as to maintain a one-phase ON-state and a two-phase ON-state for the current detection necessary time to perform the current detection.